arXiv subjects

Beatriz Noheda

Publications and source records attributed to Beatriz Noheda.

At least 19 recordsLinked to original sources

arXiv2026

20 ps Non-Destructive Read and 1 ns Write Operations at <5 V in Ferroelectric HfO2/ZrO2 Non-Volatile Memories

Achieving low-voltage, nanosecond multi-level programming and non-destructive read-out of ferroelectric non-volatile memories (NVM) is critical for analog in-memory computing architectures relying on ferroelectric capacitive devices (FeCap). We integrate HfO2/ZrO2 ferroelectric nanolayers concurrently in the BEOL of CMOS and on SiO2/Si, achieving nanosecond multilevel switching with programming voltages below 5 V. Partial ferroelectric switching enhances FeCap endurance above 1011 cycles, leading to MemCapacitance (MC) states with non-destructive read-out and 10-year retention. However, experiments reveal the collapse of the MC window for read frequencies above 1 MHz. To overcome this speed limit, we introduce a novel, non-destructive readout methodology. Using electrical pulses with widths down to 20 ps, below the RC time constant of the FeCaps, we enable measurement of the polarization-dependent leakage current, providing ultrafast and non-destructive read operations at only 14 fJ.

physics.app-ph↗

arXiv2026

PdNeuRAM: forming-free, multi-bit Pd/HfO2 ReRAM for energy-efficient neuromorphic computing

Memristor technology shows great promise for energy-efficient computing, yet it grapples with challenges like resistance drift and inherent variability. For filamentary Resistive RAM (ReRAM), one of the most investigated types of memristive devices, the expensive electroforming step required to create conductive pathways results in increased power and area overheads and reduced endurance. In this study, we present novel HfO2-based forming-free ReRAM devices, PdNeuRAM, that operate at low voltages, support multi-bit functionality, and display reduced variability. Through a deep understanding and comprehensive material characterization, we discover the key process that allows this unique behavior: a Pd-O-Hf configuration that capitalizes on Pd innate affinity for integrating into HfO2. This structure actively facilitates charge redistribution at room temperature, effectively eliminating the need for electroforming. Moreover, the fabricated ReRAM device provides tunable resistance states for dense memory and reduces programming and reading energy by 43% and 73%, respectively, using spiking neural networks (SNN). This study reveals novel mechanistic insights and delineates a strategic roadmap for the realization of power-efficient and cost-effective ReRAM devices.

cond-mat.mtrl-sci↗

arXiv2026

Disentangling the ferroelectric phases of epitaxial hafnia

Since its discovery, ferroelectric hafnia has been extensively studied due to its CMOS-compatibility and ability to remain polarized at sub-10 nm thicknesses. The ferroelectric behaviour is generally attributed to a polar orthorhombic (OIII) phase. However, a second polar phase with rhombohedral symmetry (R-phase) has also been reported in epitaxial films. The nature of the R-phase remains disputed due to the subtle differences with the OIII-phase when probed by standard thin film characterisation techniques. Given the functional properties of ferroelectrics are crucially determined by the crystal symmetry, resolving this matter is imperative. In this work, we settle the controversy through extensive 3D reciprocal space surveys made possible via synchrotron-based grazing incidence diffraction from epitaxial films of both phases. These experiments, together with direct comparison of their temperature dependence and electrical responses, conclusively establish them as two distinct phases and provide insight into their key characteristics.

cond-mat.mtrl-sci↗

arXiv2026

Device-area selection of memristive transport regimes in epitaxial $Hf_{0.5}Zr_{0.5}O_{2}$-based ferroelectric devices

Ferroelectric memristive devices based on hafnia are promising systems for neuromorphic electronics, yet the interplay between polarization-modulated resistive changes and defect-mediated transport often leads to complex and debated switching mechanisms. Here, we investigate this competition in epitaxial Hf$_{0.5}$Zr$_{0.5}$O$_2$/La$_{0.67}$Sr$_{0.33}$MnO$_3$ heterostructures with Pt top electrodes by combining structural, ferroelectric, and memristive characterization with a statistical analysis across a broad range of device areas spanning three orders of magnitude. We identify two distinct memristive regimes with opposite resistance--voltage chiralities. Small devices exhibit a low-resistance state that scales inversely with area, consistent with area-distributed tunneling transport, while larger devices display an area-independent resistance indicative of localized conductive channels. A statistical nucleation model quantitatively captures this behavior and yields a crossover characteristic area $A^* \approx 10^3~μ\mathrm{m}^2$. This crossover also correlates with the onset of ferroelectric wake-up in larger devices, linking conductive-channel nucleation and oxygen-vacancy redistribution within a unified physical picture. These results establish lateral device size as a key parameter controlling the dominant transport mechanism in epitaxial hafnia-based devices.

cond-mat.mtrl-sci↗

arXiv2025

Epitaxial Sr(Sn, Ge)$_{x}$Ti$_{1-x}$O$_{3}$ buffer layers for continuous strain engineering on SrTiO$_{3}$ substrates

Epitaxial strain plays a key role in determining the structure and functionality of thin films, with the choice of substrate being traditionally used to control the magnitude of the applied strain. However, even in the large family of perovskite materials, this allows for only a limited, discrete set of strain states to be achieved. Here we report on an approach to controlling epitaxial strain for the growth of perovskite materials by involving a single SrTiO$_{3}$ substrate (the most available perovskite in single crystal form) and a buffer layer that consists of the solid solution Sr(Sn, Ge)$_{x}$Ti$_{1-x}$O$_{3}$, of which the lattice parameter can be tuned in a continuous fashion, from 3.880 Å up to 4.007 Å, while maintaining coherent epitaxial growth on SrTiO$_{3}$ with high quality interfaces. Using a BaTiO$_{3}$ overlayer as a model system, we show that changes to the buffer layer composition, i.e. increase of in-plane lattice parameter, change the strain state of BaTiO$_{3}$ from fully relaxed, through highly compressively strained, to an exotic state showing 'inverted' epitaxy in which the buffer layer is relaxed from the substrate but lattice matched to the overlayer.

cond-mat.mtrl-sci↗

arXiv2025

Polarity and anti-distortive polarons in WO3 through epitaxial shear strain

Bestowing CMOS-compatible binary oxides with additional functionalities is a powerful strategy toward the realization of oxide electronics. Ideal candidates are thin films which display a strong sensitivity to strain, chemical doping or nanoscale confinement. Among these, crystalline tungsten trioxide WO3 exhibits exceptional structural flexibility, enabling a wide range of functionalities. Here, we reveal the emergence of a previously unreported polar phase in epitaxial WO3 thin films. We accomplish this by imposing epitaxial shear strain, which stabilizes a low-symmetry triclinic structure that persists up to large film thicknesses and elevated temperatures. At the atomic scale, a change in the oxygen octahedral tilt pattern facilitates this symmetry lowering into a polar phase, which manifests as a periodic in-plane polarized stripe domain configuration with needle-like bifurcations at the microscale. The stripe domain walls further exhibit a strongly enhanced electrical conductivity in conjunction with a pronounced reduction of a distortive structural mode, providing the first experimental evidence for the formation of anti-distortive polarons recently predicted in WO3.

cond-mat.mtrl-sci↗

arXiv2025

Integration of imprint-free and low coercivity ferroelectric BaTiO3 thin films on silicon

Highly-crystalline ferroelectric oxides integrated on Si hold great promise for energy-efficient memory and logic technologies. Exploiting epitaxial strain engineering in these materials is, however, severely hampered on Si, where the large structural mismatch often results in an inferior interfacial quality and causes a degradation of the ferroelectric switching characteristics. In this work, we present the growth of single-crystalline BaTiO3 thin films on Si, exhibiting imprint-free switching, low coercivity, high remanent polarization, and no fatigue for over $10^{10}$ switching cycles. We accomplish this via the insertion of a SrSn1-xTixO3 layer on SrTiO3-buffered Si. This layer serves as a pseudo substrate that alleviates the thermal strain that the Si substrates imposes on the BaTiO3 layer, while simultaneously providing moderate compressive strain that stabilizes a pure out-of-plane polarization. Thus, our work paves the way toward the fabrication of Si-compatible, low-power-consuming ferroelectric devices for non-volatile memory applications.

cond-mat.mtrl-sci↗

arXiv2025

Millisecond-scale Volatile Memory in HZO Ferroelectric Capacitors for Bio-inspired Temporal Computing

With the broad recent research on ferroelectric hafnium oxide for non-volatile memory technology, depolarization effects in HfO2-based ferroelectric devices gained a lot of interest. Understanding the physical mechanisms regulating the retention of these devices provides an excellent opportunity for device optimization both towards non-volatile memory applications and towards real-time signal processing applications in which controlled time constants are of paramount importance. Indeed, we argue that ferroelectric devices, particularly HfO2-based, are an elegant solution to realize possibly arbitrary time constants in a single scaled memory device, which paves the way for temporal and brain-inspired computing in hardware. Here we present a ferroelectric capacitor stack realizing volatile memory due to its unique interface configuration. We provide electrical characterization of the device to motivate its use for realizing time constants in hardware, followed by an investigation of the electronic mechanisms and their possible relation to the observed retention times to facilitate further modeling of the retention process in HfO2-based ferroelectric capacitors. In the presented device, internal electric fields stabilize one polarization of the ferroelectric film, opening the possibility for unipolar operation with millisecond retention for the unstable polarization state. We show a dependence of the retention on both the polarization as well as the electrical stimuli, allowing us to exploit a range of time scales in a single device. Further, the intentionally defective interface in the presented material stack allows an insight into the interplay between retention loss in HfO2-based ferroelectric devices and the internal bias field, which we relate to the interface composition and the role of oxygen vacancies as a possible source of the internal bias fields.

cs.ET↗

arXiv2025

Guidelines for the optimization of hafnia-based ferroelectrics through superlattice engineering

Hafnia-based ferroelectrics are revolutionizing the data storage industry and the field of ferroelectrics, with improved materials and devices being reported monthly. However, full understanding and control has not been reached yet and the ideal material still needs to be found. Here we report ferroelectric hafnia-zirconia superlattices made out of zirconium-substituted hafnia (Hf$_{1-x}$Zr$_x$O$_2$) sublayers of varying stoichiometries alternating with pure ZrO$_2$ sublayers. It is observed that the ZrO$_2$ layers in these superlattices act as a booster for the total remnant polarization (P$_r$). By combining the benefits of the ZrO$_2$ layers and the added interfaces, which help prevent breakdown, we fabricate superlattices with a total 87.5% ZrO$_2$ content, exhibiting record polarizations with a 2P$_r$ value of 84 $μ$C/cm$^2$ that can be cycled 10$^9$ times, while maintaining a 2P$_r$ > 20 $μ$C/cm$^2$. Next to these attractive properties, substitution of HfO$_2$ by the much more abundant ZrO$_2$ offers a significant step towards the sustainable application of these devices.

cond-mat.mtrl-sci↗

arXiv2025

Oxygen vacancy engineering in pulsed laser deposited BaSnO$_3$ thin films on SrTiO$_3$

We demonstrate the tunability of oxygen content in pulsed laser deposition (PLD)-grown barium stannate (BaSn$O_3$, BSO) thin films by precisely controlling the background oxygen pressure over a broad range from 0.0004 mbar to 0.13 mbar. The introduction of oxygen vacancies significantly alters the structural properties of BSO films, inducing a monotonic expansion of the out-of-plane lattice parameter and cell volume as the vacancy concentration increases. The progressive formation of oxygen vacancies was spectroscopically tracked using X-ray photoelectron spectroscopy (XPS), providing direct insight into the vacancy evolution. Furthermore, we show that the oxygen stoichiometry in BSO plays a critical role in modulating the sheet resistance of BSO/LaScO$_3$ heterostructures, enabling interface metallic electron conduction. This oxygen content control offers a robust strategy to tailor the electronic properties at the interface, highlighting its potential for oxide electronics and functional interface engineering.

cond-mat.mtrl-sci↗

arXiv2024

Roadmap to Neuromorphic Computing with Emerging Technologies

The roadmap is organized into several thematic sections, outlining current computing challenges, discussing the neuromorphic computing approach, analyzing mature and currently utilized technologies, providing an overview of emerging technologies, addressing material challenges, exploring novel computing concepts, and finally examining the maturity level of emerging technologies while determining the next essential steps for their advancement.

eess.SP↗

arXiv2024

Multi-mem behavior at reduced voltages in La$_{1/2}$Sr$_{1/2}$Mn$_{1/2}$Co$_{1/2}$O$_{3-x}$ perovskite modified with Sm:CeO$_2$

Neuromorphic computing aims to mimic the architecture and the information processing mechanisms of the mammalian brain, appearing as the only avenue that offers significant energy savings compared to the standard digital computers. Memcapacitive devices (which can change their capacitance between different non-volatile states upon the application of electrical stimulation) can significantly reduce the energy consumption of bioinspired circuitry. In the present work, we study the multimem (memristive and memcapacitive) behavior of devices based on thin films of the topotactic redox La$_{1/2}$Sr$_{1/2}$Mn$_{1/2}$Co$_{1/2}$O$_{3-x}$ (LSMCO) perovskite modified with Sm:Ce$O_2$ (SCO), grown on Nb:SrTiO$_{3}$ with (001) and (110) out of plane orientations. Either the self assembling at the nanoscale of both LSMCO and SCO phases or the doping with Ce(Sm) of the LSMCO perovskite were observed for different fabrication conditions and out of plane orientations. The impact of these changes on the device electrical behavior was determined. The optimum devices resulted those with (110) orientation and Ce(Sm) doping the perovskite. These devices displayed a multimem behavior with robust memcapacitance and significantly lower operation voltages (especially the RESET voltage) in comparison with devices based on pristine LSMCO. In addition, they were able to endure electrical cycling (and the concomitant perovskite topotactic redox transition between oxidized and reduced phases) without suffering nanostructural or chemical changes. We link these properties to an enhanced perovskite reducibility upon Ce(Sm) doping. Our work contributes to increase the reliability of LSMCO based multimem systems and to reduce their operating voltages closer to the 1 V threshold, which are key issues for the development of nanodevices for neuromorphic or in memory computing.

physics.app-ph↗

arXiv2023

Toward a formal theory for computing machines made out of whatever physics offers: extended version

Approaching limitations of digital computing technologies have spurred research in neuromorphic and other unconventional approaches to computing. Here we argue that if we want to systematically engineer computing systems that are based on unconventional physical effects, we need guidance from a formal theory that is different from the symbolic-algorithmic theory of today's computer science textbooks. We propose a general strategy for developing such a theory, and within that general view, a specific approach that we call "fluent computing". In contrast to Turing, who modeled computing processes from a top-down perspective as symbolic reasoning, we adopt the scientific paradigm of physics and model physical computing systems bottom-up by formalizing what can ultimately be measured in any physical substrate. This leads to an understanding of computing as the structuring of processes, while classical models of computing systems describe the processing of structures.

cs.ET↗

arXiv2023

Dynamics of Voltage Driven Self-Sustained Oscillations in NdNiO$_3$ Neuristors

Active memristor elements, also called neuristors, are self-oscillating devices that are very good approximations to biological neuronal functionality and are crucial to the development of low-power neuromorphic hardware. Materials that show conduction mechanisms that depend superlinearly with temperature can lead to negative differential resistance (NDR) regimes, which may further be engineered as self-oscillators. Thermal runaway, insulator to metal phase transitions (IMT) can lead to such superlinearity and are being extensively studied in systems such as TaO$_x$, NbO$_x$ and VO$_2$. However, ReNiO$_3$ systems that offer large tunability in metal-insulator transition temperatures are less explored so far. Here we demonstrate all-or-nothing neuron-like self-oscillations at MHz frequency and low temperatures on thin films of NdNiO$_3$, a model charge transfer insulator, and their frequency coding behavior. We study the temperature dependence of NDR and show that it vanishes even at temperatures below the IMT temperature. We also show that the threshold voltages scale with device size and that a simple electrothermal device model captures all these salient features. In contrast to existing models, our model correctly predicts the independence of oscillation amplitude with the applied voltage, offering crucial insights about the nature of fixed points in the NDR region, and the dynamics of non-linear oscillations about them. KEYWORDS: NDR, oscillations, thermal model.

physics.app-ph↗

arXiv2022

Investigating the Electromechanical Behavior of Unconventionally Ferroelectric $Hf_{0.5}Zr_{0.5}O_{2}$-based Capacitors Through Operando Nanobeam X-ray Diffraction

Understanding various aspects of ferroelectricity in hafnia-based nanomaterials is of vital importance for the development of future non-volatile memory and logic devices. Here, the unconventional and weak electromechanical response of epitaxial $La_{0.67}Sr_{0.33}MnO_{3}$/$Hf_{0.5}Zr_{0.5}O_{2}$/$La_{0.67}Sr_{0.33}MnO_{3}$ ferroelectric capacitors is investigated, via the sensitivity offered by nanobeam X-ray diffraction experiments during application of electrical bias. It is shown that the pristine rhombohedral phase exhibits a negative linear piezoelectric effect with piezoelectric coefficient ($d_{33}$) ~ -0.5 to -0.8 pm$V^{-1}$. First-principles calculations support an intrinsic negative piezoresponse. In addition, it is found that the piezoelectric response is suppressed above the coercive voltage. For higher voltages, and with the onset of DC conductivity throughout the capacitor, a second-order effect is observed. The electromechanical response observed in this work is clearly different from that of normal ferroelectrics, again underlining the unconventional nature of polarization switching in the samples.

cond-mat.mtrl-sci↗

arXiv2022

Dynamic Tilting of Ferroelectric Domain Walls via Optically Induced Electronic Screening

Optical excitation perturbs the balance of phenomena selecting the tilt orientation of domain walls within ferroelectric thin films. The high carrier density induced in a low-strain BaTiO3 thin film by an above-bandgap ultrafast optical pulse changes the tilt angle that 90° a/c domain walls form with respect to the substrate-film interface. The dynamics of the changes are apparent in time-resolved synchrotron x-ray scattering studies of the domain diffuse scattering. Tilting occurs at 298 K, a temperature at which the a/b and a/c domain phases coexist but is absent at 343 K in the better ordered single-phase a/c regime. Phase coexistence at 298 K leads to increased domain-wall charge density, and thus a larger screening effect than in the single-phase regime. The screening mechanism points to new directions for the manipulation of nanoscale ferroelectricity.

cond-mat.mtrl-sci↗

arXiv2022

Phenomenological classification of metals based on resistivity

Efforts to understand metallic behaviour have led to important concepts such as those of strange metal, bad metal or Planckian metal. However, a unified description of metallic resistivity is still missing. An empirical analysis of a large variety of metals shows that the parallel resistor formalism used in the cuprates, which includes T-linear and T-quadratic dependence of the electron scattering rates, can be used to provide a phenomenological description of the electrical resistivity in all metals, where these two contributions are shown to correspond to the two first terms of a Taylor expansion of the resistivity, detached of their physics origin, and thus, valid for any metal. Here we show that the different metallic classes are then determined by the relative magnitude of these two components and the magnitude of the extrapolated residual resistivity. These two parameters allow to categorize a few systems that are notoriously hard to ascribe to one of the currently accepted metallic classes. This approach also reveals that the T-linear term has a common origin in all cases, strengthening the arguments that propose the universal character of the Planckian dissipation bound.

cond-mat.str-el↗

arXiv2022

Unusual ferrimagnetism in CaFe2O4

Incomplete cancellation of collinear antiparallel spins gives rise to ferrimagnetism. Even if the oppositely polarized spins are owing to the equal number of a single magnetic element having the same valence state, in principle, a ferrimagnetic state can still arise from the crystallographic inequivalence of the host ions. However, experimental identification of such a state as ferrimagnetic is not straightforward because of the tiny magnitude expected for M and the requirement for a sophisticated technique to differentiate similar magnetic sites. We report a synchrotron-based resonant x-ray investigation at the Fe L2,3 edges on an epitaxial film of CaFe2O4, which exhibits two magnetic phases with similar energies. We find that while one phase of CaFe2O4 is antiferromagnetic, the other one is ferrimagnetic with an antiparallel arrangement of an equal number of spins between two distinct crystallographic sites with very similar local coordination environments. Our results further indicate two distinct origins of an overall minute M; one is intrinsic, from distinct Fe3+ sites, and the other one is extrinsic, arising from defective Fe2+ likely forming weakly-coupled ferrimagnetic clusters. These two origins are uncorrelated and have very different coercive fields. Hence, this work provides a direct experimental demonstration of ferrimagnetism solely due to crystallographic inequivalence of the Fe3+ as the origin of the weak M of CaFe2O4.

cond-mat.mtrl-sci↗